Propellant formed cure-shrinkable propellant material

ABSTRACT

This patent describes a novel caseless ammunition comprising a solid unitary propellant grain of generally uniform circular cross-section and containing a plurality of longitudinal openings therein extending the length of said grain to provide increased burning surface. The ammunition is prepared by adding a flowable, curable propellant to a generally enclosed mold of circular cross-section containing a plurality of longitudinally extending pins, curing said propellant, and removing the mold including the pins. This patent also discloses a novel solid rocket motor comprising a plurality of spaced-apart free-standing propellant grains cast in a rocket motor casing. A method for forming a novel propellant grain structure in a motor chamber which comprises inserting a separator which divides the chamber into a plurality of smaller chambers, filling the chamber with a propellant which shrinks upon curing, and curing the propellant. A method for forming a novel propellant grain structure in a motor chamber which comprises casting the propellant, chosen from a class which has the characteristic of shrinking upon curing, around lengthwise positioned mandrels coated with a mold release agent, curing the propellant, removing the mandrels to provide spaces in the structure and casting and curing additional propellant in the spaces provided by the mandrels.

United States Patent [1 Cohen et al.

154] PROPELLANT FORMED CURE-SHRINKABLE PROPELLANT MATERIAL [75] inventors: Joseph Cohen, Carmichael, Calif.;

Ralph Scotoni, Jr., Eifeld, Switzerland [73] Assignee: Aerojet-General Corporation, El

Monte, Calif.

[22] Filed: Mar. 30, 1970 [21] Appl. No.: 23,895

Related US. Application Data [63] Continuation-impart of Ser. Nos. 384,821, July 21, 1964, abandoned, and Ser. No. 790,180, June 28, 1968, abandoned.

[52] US. Cl 102/100, 60/255, 60/256, 102/101, 149/2, 149/14, 149/88, 149/89, 149/93, 149/98, 264/3 R [51] Int. Cl. F421) l/00 [58] Field of Search 60/253, 254, 255, 256; 102/100, 101; 264/3 R [5 6] References Cited UNITED STATES PATENTS 2,488,154 11/1949 Africano 60/255 2,502,458 4/1950 Hickman 60/255 2,939,275 6/1960 Loedding 60/255 3,023,570 3/1962 Crouch 60/255 3,140,663 7/1964 Rumbel et al.... 264/3 R UX 3,191,535 6/1965 Mulloy 264/3 R X 3,252,369 5/1966 Bartley .1 264/3 R X 3,413,384 11/1968 Olliff, Jr 264/3 R X 1 May 28, 1974 Primary Examiner-Leland A. Sebastian Attorney Agent, or F irm-E. O. Ansell, C. Jacobs [57] ABSTRACT This patent describes a novel caseless ammunition comprising a solid unitary propellant grain of generally uniform circular cross-section and containing a plurality of longitudinal openings therein extending the length of said grain to provide increased burning surface. The ammunition is prepared by adding a flowable, curable propellant to a generally enclosed mold of circular cross-section containing a plurality of longitudinally extending pins, curing said propellant, and removing the mold including the pins.

This patent also discloses a novel solid rocket motor comprising a plurality of spaced-apart free-standing propellant grains cast in a rocket motor casing. A method for forming a novel propellant grain structure in a motor chamber which comprises inserting a separator which divides the chamber into a plurality of smaller chambers, filling the chamber with a propellant which shrinks upon curing, and curing the propellant. A method for forming a novel propellant grain structure in a motor chamber which comprises casting the propellant, chosen from a class which has the characteristic of shrinking upon curing, around lengthwise positioned mandrels coated with a mold release agent, curing the propellant, removing the mandrels to provide spaces in the structure and casting and curing additional propellant in the spaces provided by the mandrels.

21 Claims, 9 Drawing Figures 'FAIENTEnmwm i 3312.785 mum INVENTORS JOSEPH COHEN RALPH SOOTONLJR.

AAAAAA YS FATENTEBMAY 23 1914 SHEET 2 BF 4 s JOSEPH c RALPH OTONI, JR.

f, o g 0 INVEN ATTORNEYS minnow m4 3812.785

SHEET ll 0F 4 INVENTORS JOSEPH COHEN RALPH SCOTONI, JR.

PROPELLANT FORMED CURE-SHRINKABLE PROPELLANT MATERIAL This is a continuation-in-part of application Ser. No. 384,821, filed July 21, 1964, now abandoned, and 790,180, filed June 28, 1968, now abandoned.

BACKGROUND OF THE INVENTION This inventionrelates to the improvement of the reliability and combustion of solid propellants used in caseless ammunition and solid rocket motors.

The mass burning rate of a propellant, whether in the instance of caseless ammunition or a solid rocket motor is a function of the available burning area, all other parameters being held constant. Thus, the available burn- .ing area has always been a limiting factor in the development of a large size caseless ammunition or high thrust rocket motors which must have a high mass flow rate and a short burning time. The difficulty, in general, in previously proposed solutions to this problem has been the attendant loss of structure integrity under severe shock or stress conditions. It is believed that this invention substantially overcomes this problem, and represents a major advance in the art.

SUMMARY OF THE INVENTION Briefly, the present invention comprises a novel caseless ammunition comprising a solid unitary propellant grain of generally uniform circular cross-section and containing a plurality of longitudinal openings therein extending the length of said grain to provide increased burning surface. The ammunition is prepared by adding a flowable, curable propellant to a generally enclosed mold of circular cross-section containing a plurality of longitudinally extending pins, curing said propellant, and removing the mold including the pins.

This invention also includes a novel solid rocket motor comprising a plurality of spaced-apart freestanding propellant grains cast in a rocket motor casing. This patent further encompasses a method for forming a novel propellant grain structure in a motor chamber whichcomprises inserting a separator which divides the chamber into a plurality of smaller chambers, filling the chamber with a propellant which shrinks (from about 0.07 to 3.3 percent, preferably 1.0 to 3.3 percent) upon curing, and curing the propellant. Still further, this invention includes a method for forming a novel propellant grain structure in a motor chamher which comprises casting a propellant, chosen from a class which has the characteristic of shrinking upon curing, around lengthwise positioned mandrels coated with a mold release agent, curing the propellant, removing the mandrels to provide spaces in the structure and casting and curing additional propellant in the spaces provided by the mandrels.

It is an object of this invention to provide a propellant charge for caseless ammunition and rocket motors with a high mass flow rate and with a low linear burning rate.

It is also an object of the present invention to produce grains for caseless ammunition and rocket motors with sufficient voids to relieve stress concentrations.

It is a further object of this invention to provide a propellant configuration which will have a high mass flow and a short burning time. effective for caseless ammunition and high acceleration rocket motors.

Further objects and additional advantages of the invention will become apparent from the following detailed description and annexed drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS Turning to the drawings:

FIG. I is a fragmentary plan view of a propellant configuration according to the present invention;

FIG. 2 is a cross-sectional view taken on lines 22 of FIG. 1, shown slightly enlarged;

FIG. 3 is a cross-sectional view taken on lines 3-3 of FIG. 5 of a motor chamber showing another configuration;

FIG. 4 is a cross-sectional view taken on lines 4-4 of FIG. 3, shown slightly enlarged;

FIG. 5 is an overall view of a rocket generally;

FIG. 6 is a fragmentary plan view of another propellant configuration of this invention;

FIG. 7 is a cross-sectional view taken on lines 7-7 of FIG. 6;

FIG. 8 shows in perspective view the mold used in forming caseless ammunition; and

FIG. 9 shows the caseless ammunition after removal of the mold.

Referring now to the drawings in greater detail, there is shown in FIG. I, a plurality of spaced apart free standing propellant grains 10, arranged in the shape of hexagonal cells. Separators 12, which are disposed between the grains and used to form the individual cells are a thin, continuous, solid material which will be described in greater detail hereinafter. The grains are completely cast in the motor chamber and are all bonded to the chamber itself, either to the head, end or sidewall.

FIG. 2 shows individual propellant grains 10, with separators l2 bonded to the motor case 14 by liner 16. The grains may extend either axially or.radially in the motor chamber and may be single standing grains or placed in stacks on top of each other as shown later in FIG. 6.

FIG. 3 shows another configuration of propellant grains 18, arranged axially as hexagonal cells by separators 20. Each individual grain contains a hole 22, down the center provided upon casting by a removable center pin (not shown). The grains are bonded to the sides of the motor case 24 by liner 26.

FIG. 4 shows the individual standing grains 18, each with a center hole 22. Separators 20 separate the grains which are bonded to the bottom of the motor case 28 by line 26.

FIG. 5 shows a rocket 30 with nose 32, nozzle 34, and

' motor case 36.

FIG. 6 shows an alternative propellant configuration in which the individual grains 38, separated by separators 40, are stacked in rows provided by the honeycomb shape of the separator.

FIG. 7 shows the individual standing grains 38 of FIG. 6 with separators 40.

The configuration of FIG. 6 can be modified by overloading each successive layer or row of grains to provide a flat layer of propellant between each row of grains. This type of configuration gives a high motor loading at lower thrust capability. In addition, the configuration of FIG. 6 can be modified by placing a separator or membrane capable of stopping the burning, such as a cellulose acetate membrane. between each row of grains. In this manner a controlled pulse-type burning capability is obtained since the burning is stopped at each separator or membrane and the motor may be easily reignited when desired.

FIG. 8 shows the mold 42 having an outer case 44 and a plurality of longitudinally extending internal pins 46.

FIG. 9 shows the caseless ammunition prepared in mold 42 after the propellant has been cured and the mold and pins 46 removed.

The novel configurations of the present invention may be provided by either of two methods.

METHOD 1 The method of preparing caseless ammunition according to this invention can be carried out using, for example, the mold 42. The mold 42 and pins 46 are first coated with a mold release agent. The propellant slurry is then added to the mold. Two different propellant formulations were used having the following composition:

Trinicthylolmcthane trinitrate 31.0 Nitroglyccrin 43.8 9.0 Nitrocellulose. Fluid Bull Powder, 45.6 50,0

type Triacetin 4.3 4.5 Adiponilrilc 4.3 4.5 LL oxide Complete packing and the elimination of bubbles can be obtained by the use of a vibrating table during filling. The filled mold is placed in an oven to cure the propellant after which the mold and pins are removed to provide caseless ammunition of the type shown in H0. 9.

METHOD 2 The solid propellant is cast either axially or radially with separators. The separators divide the rocket motor chamber into a multitude of smaller chambers. Upon filling the motor with propellant, each small chamber contains an individual grain. This is accomplished by the shrinkage of the propellant upon curing which creates free standing grains with one end being bonded to the case. During firing of the motor, each small propellant grain inside each chamber created by the separators burns on its side as well as on the unbonded end thereby creating a very high mass flow.

Curing is conducted at temperatures ranging from ambient to 220F. If the lower termperatures are used, the charge requires a longer time to cure. lf a shorter time is desired. the more elevated temperatures can be used.

The separators, which may be made in any geometrical shape desired, such as in the form of hexagonal cells as illustrated in H6. 1, are coated with a mold release agent, so that when the propellant is cured, it easily shrinks away from the separator. The separator may be removed from the motor chamber, however, it is preferred to leave the separator in, since its presence alleviates resonant burning.

METHOD 3 A first casting of propellant is prepared around lengthwise positioned mandrels which have been coated with a mold release agent. After the propellant has been cured, the mandrels are withdrawn and the holes are coated with a mold release agent. A second propellant is then cast in these holes. Upon cure and shrinkage of the second propellant, voids are left between the first and second propellant to give increased burning surface area. The two propellants used for this method may be the same or different.

Any solid propellant that contracts upon cure, preferably on the order of about two to about three percent, is useful for the rocket motor application of this invention. However, shrinkage during cure is not important, in fact, can be a disadvantage in the forming of caseless gun ammunition. In this instance, shrinking around the pin-shaped-mandrels has the effect of making the pin removal more difficult. In the rocket application, shrinking during cure is preferred to provide for rapid progression of the flame front by providing open passages along the honeycomb or other spacers. Solid propellant compositions are ordinarily composed of a resin fuel and an oxidizing material, the oxidizing material being intimately dispersed in the fuel. Hence, the invention is broad enough to encompass the field of solid propellants in general, including those containing well known binders, such as nitropolyurethane, polyesteracrylate, rubber (butyl, polysulfide), nitrocellulose, etc.

Propellants preferred for the purposes of this invention include polyester, epoxy, and double based propellants. For example, the resin comprises a polyester component, that is, the reaction product of a polycarboxylic acid with a polyhydric alcohol with which there is incorporated a monomeric olefin component such as a vinyl, allyl or other olefin compatible with a liquid resin. A polyester component, sometimes known as an alkyd component or an alkyd resin should possess some degree of unsaturation in the molecule in order to permit it to heteropolymerize with the olefinic component which also possesses unsaturation and which may be, for example: phenyl substituted lower alkenes such as styrene; lower alkenyl esters of lower alkanoic acids, such as vinyl acetate; lower alkyl esters of lower alkanoic acids, such as esters of acrylic or methacrylic acid; allyl compounds such as allyl diglycol carbonate; lower alkenyl esters of lower alkanoic acids, such as diallyl malleate; diallyl diglycollate. Other olefinic components include lower alkyl dienes such as butadiene, lower alkynes such as acetylene, etc.; and derivatives of any of the above substances which will polymerize with the resin. In general, any olefin compatible with the resin and which will polymerize with it is suitable. This includes essentially all unsubstituted olefins and in addition, many substituted olefins.

The alcohols that may be used include the dihydric alcohols as well as other polyhydric alcohols such as the trihydric and higher polyhydric alcohols. Specifically, alcohols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, glycerol, erythritols, pentaerythritol, arabitol, adonitol, xylitol, mannitol, sorbitol, dulcitol, persitol, and volamitol. Mixtures of the above alcohols may also be employed if desired.

Similary, epoxy type propellants which may be used for the purposes of the invention include a polymeric epoxy component as the resin material.

Double based propellants which may be used for the purposes of this invention include, for example, a binder material such as nitrocellulose with a mixture of a nitrate compound such as nitroglycerin and a material such as triacetin or adiponitrile serving as the plasticizer.

Nitrate compounds other than nitroglycerin which may be used in propellants of this invention include nitrato esters of the aliphatic polyhydric alcohols, the esters having a molecular weight of from about I00 to about 1,000. More specifically, the nitrate esters are those having the general formula:

wherein n is an integer from two to about five and R is saturated aliphatic hydrocarbon radical having a valence equal to n. Normally R contains from one to about carbon atoms. Illustrative of compounds of the above formula are nitroglycerin, diethylene glycol dinitrate, dipropylene glycol dinitrate, triethylene glycol dinitrate, trimethylol ethane trinitrate, pentaerythritol trinitrate, pentaerythritol tetranitrate, or mixtures thereof.

Nitro compounds which may be used in place of the triacetin or adiponitrile include any nitro compounds which are miscible with the above-mentioned nitrato compounds, and have a pH of from about 6 to about 8. Illustrative of suitable nitro compounds are the nitroalkenes such as nitromethane, l,l-nitroethane and hexanitroethane; and the nitroalkanols such as delta trinitroethanol, gem-dinitropropanol and 2,2- dinitrodecanol. The nitroalkanes and nitroalkanols normally contain from one to about 10 carbon atoms and from one to about eight nitro groups per molecule.

Another class of nitro compounds useful with this type of propellant composition are the nitroacetals having the following general formula:

- kyl, haloalkyl, or nitrazaalkyl radical; R is a hydrogen,

alkyl, halogen, nit r0alkyl, nitro, aryl, alkaryl, arylalkyl, cycloalkyl, haloalkyl, or nitrazaalkyl radical which may i or may not be the same as R and A is an alkylene radical. These compounds and their'preparation are disclosed in greater detail in assignees copending U.S. application Ser. No. 2,072, filed Jan. 12, I960, now abandoned. Preferred compounds within the scope of this formula are bis-dinitropropyl formal; (prepared from 2,2-dinitropropanol and formaldehyde), bisdinitropropyl acetal (prepared from 2,2- dinitropropanol and acetaldehyde), bis-( 2,2,2-

trinitroethyl) acetal, pentyl)acetal.

and bis-(2,2,4,4-tetranitro-nof a non-energetic coplasticizer such as diethylphthalate, polydiethylene glycol adipate, adiponitrile, dibutylphthalate, triacetoglycerol, tributyro-glycerol and mixtures thereof.

In preparing propellants containing nitrato compound-nitro compound mixtures, the mixture is employed in the propellant in an amount from about 20 percent to about 90 percent by weight based on the total weight of the propellant composition. The binder material is used in an amount of from about 5 percent to about 70 percent by weight based on the total weight of the propellant composition.

Any compatible polymeric material may be used as the binder, Preferred binders are nitrocellulose and the polyurethanes. The nitrocellulose binders are normally polymers having a molecular weight of from about 5,000 to about 5,000,000, a nitrogen content from about 1 1.9 percent to about 12.9 percent by weight and a density from about L5 to 1.55 grams per cc. Preferred nitrocellulose for use in these propellants has a particle size from about 5 to about 100 microns.

In addition to the nitrato compound-nitro compound mixture and the binder, the balance of the propellant composition up to about 50 percent by weight can be other conventional propellant ingredients such as solid oxidizers; e.g.,' ammonium perchlorate, hydrazine nitrate, hydrazine perchlorate, nitroguanidine, cyclotrimethyltrinitramine, cyclotetramethyltetranitramine, as well as solid fuels such as powdered aluminum, powdered beryllium, powdered zirconium, aluminum hydride, beryllium hydride, and mixtures thereof. The solid fuels are preferably used in finely divided form. In addition, the nitrocellulose-based propellants may also contain small amounts of stabilizers such as 2-nitrodiphenylamine and burning-rate additives such as lead salicylate.

Agents which are useful as mold release agents for the purposes of this invention include any material which is not reactive with or absorbed by the propellant binder. This includes such materials as silicones, oils, greases, waxes, microcrystalline waxes and paraffin.

Propellant cast in the manner of this invention is easily ignited since the constrictions give very high pressures and erosive burning. The gases given off from the propellant raise the pressure in the constrictions. Since the pressure is high in the beginning when the constrictions are small, the burning rate is highest in the beginmng.

The separator or honeycomb material useful in this invention may be any solid material which will provide the cells necessary for casting the propellant grains. Suitable materials include aluminum foil and glass cloth impregnated with a phenolic resin. Preferably the In the above formula groups R,, R R R and A preferably are the lower members of the series, i.e., .lower alkyl, lower alkylene, etc. and contain from one material is as lightweight as possible so as not to increase the load in the rocket motor.

As set forth previously, the novel propellant configuration of this invention provides a high mass flow and easy ignition. This advantage provides a means for using ammonium nitrate propellants more widely. Ammonium nitrate propellants are hard to ignite but are cheap, smokeless, and the exhaust is non-toxic and non-corrosive. When cast in the structure of this invention, ammonium nitrate propellants ignite more easily, and may be effectively used in areas where they were not previously effective.

In the following examples, parts and percentages are by weight unless indicated otherwise.

EXAMPLE I Weight 1 Nitroglycerin 42.50 Nitrocellulose 50.10 Triacetin 2.70 Adiponitrile 2.70 Lead Oxide 0.50 Acetyl Salicyclic Acid l 50 EXAMPLE ll The propellant used in Example I was cast into a mold in which a honeycomb structure composed of glass cloth impregnated with a phenolic resin had been of a cure-shrinkable material and further characterized in having free space between the separators and the free-standing grains.

2. The propellant charge of claim 1 in which a hole is present down the center of each grain, said hole having at least some longitudinally extending free space.

3. In a rocket motor, the improvement of a propellant charge containing a plurality of spaced-apart freestanding propellant grains and having a separator disposed between each of the grains, said propellant grains being formed of a cure-shrinkable material and further characterized in having free space between the separators and the free-standing grains.

4. The propellant charge of claim 3 in which a hole is present down the center of each grain, said hole having at least some longitudinally extending free space.

5. A rocket motor in accordance with. claim 3 f wherein the separator is formed of a fabric impregbonded to the base of the mold. The honeycomb was covered with a silicone grease mold release agent prior to casting and curing the propellant. Upon firing, approximately a fourfold increase in mass flow rate and a progressive pressure-time curve was observed.

EXAMPLE lll The propellant used in Example I was cast into a mold in which a honeycomb structure composed of glass cloth impregnated with a phenolic resin had been bonded to the base of the mold. A hollow core was cast into each cell grain with a removable center pin. Both the honeycomb and center pin were coated with a silicone grease mold release agent prior to casting and curing the propellant. Upon firing, approximately a tenfold increase in mass flow rate and a neutral pressure-time curve was observed.

EXAMPLE IV The propellant used in Example I was cast into cylindrical grains using removable cylinders to form the individual grains. A hollow core was cast in' each grain with a removable center pin. A single of honeycomb one inch wide and six inches long was imbedded into the surface of the grain at the core by placing the strip of honeycomb along the center pin before casting. The strip then becomes imbedded in the core of the grain upon cure. Upon firing, no resonance was observed whereas firings without the honeycomb exhibited reso-.

nance.

. EXAM PLE V The propellant used in example I was cast into the mold shown in FIG. 9. After curing and removal of the mold, satisfactory 27 mm caseless ammunition was obvnated with a phenolic resin.

6. A rocket motor in accordance with claim 3 wherein the separator is formed of aluminum foil.

7. A rocket motor in accordance with claim 1 wherein the separator is formed of a fabric impregnated with a phenolic resin.

8. A rocket motor in accordance with claim 1 wherein the separator is formed of aluminum foil.

9. In a method for forming in a rocket motor chamber a propellant grain structure having a plurality of spaced-apart free-standing propellant grains, the improvement comprising:

providing in the rocket chamber a separator structure which structure divides the interiorof the rocket chamber into a plurality of smaller chambers;

supplying the rocket chamber with a flowable cureshrin'kable propellant material; and

curing the propellant material within the rocket chamber and during curing permitting the propellant material of the several smaller chambers to shrink away from-the separator structure to form the plurality of free-standing propellant grains.

10. The method of claim 9.in which the separator is a honeycomb in the shape of hexagonal cells.

11. The method of claim 9 in which the separator is coated with a mold release agent.

12. The method of claim 10 in which the honeycomb is coated with a mold release agent.

13. The method of claim 9 in which said propellant is chosen from a class which has the characteristic of shrinking on the order of about 2 percent to about 3 percent upon curing.

14. The method of claim 13 in which the propellant is a polyester-based propellant.

15. The method of claim 13 in which the propellant is a double-based propellant.

16. A method in accordance with claim 9 wherein each of the smaller chambers defined by the separator 7 structure has a mandrel positioned therein, said mandrels being coated with a mold release agent, which mandrels following the casting and curing are removed to provide longitudinal voids in the respective freestanding propellant grains.

17. A method in accordance with claim 16 wherein additional propellant is cast and cured in the longitudinal voids of the respective freestanding propellant grains.

18. The method of claim 17 in which the spaces provided by removing the mandrels are coated with a mold release agent prior to casting additional propellant.

19. The method of claim 17 in which the propellant upon curing shrinks on the order of about 2 percent to about 3 percent.

20. The method of claim 19 in which the propellant is a polyester-based propellant.

21. The method of claim 19 in which the propellant S is a double-based propellant. l 

2. The propellant charge of claim 1 in which a hole is present down the center of each grain, said hole having at least some longitudinally extending free space.
 3. In a rocket motor, the improvement of a propellant charge containing a plurality of spaced-apart free-standing propellant grains and having a separator disposed between each of the grains, said propellant grains being formed of a cure-shrinkable material and further characterized in having free space between the separators and the free-standing grains.
 4. The propellant charge of claim 3 in which a hole is present down the center of each grain, said hole having at least some longitudinally extending free space.
 5. A rocket motor in accordance with claim 3 wherein the separator is formed of a fabric impregnated with a phenolic resin.
 6. A rocket motor in accordance with claim 3 wherein the separator is formed of aluminum foil.
 7. A rocket motor in accordance with claim 1 wherein the separator is formed of a fabric impregnated with a phenolic resin.
 8. A rocket motor in accordance with claim 1 wherein the separator is formed of aluminum foil.
 9. In a method for forming in a rocket motor chamber a propellant grain structure having a plurality of spaced-apart free-standing propellant grains, the improvement comprising: providing in the rocket chamber a separator structure which structure divides the interior of the rocket chamber into a plurality of smaller chambers; supplying the rocket chamber with a flowable cure-shrinkable propellant material; and curing the propellant material within the rocket chamber and during curing permitting the propellant material of the several smaller chambers to shrink away from the separator structure to form the plurality of free-standing propellant grains.
 10. The method of claim 9 in which the separator is a honeycomb in the shape of hexagonal cells.
 11. The method of claim 9 in which the separator is coated with a mold release agent.
 12. The method of claim 10 in which the honeycomb is coated with a mold release agent.
 13. The method of claim 9 in which said propellant is chosen from a class which has the characteristic of shrinking on the order of about 2 percent to about 3 percent upon curing.
 14. The method of claim 13 in which the propellant is a polyester-based propellant.
 15. The method of claim 13 in which the propellant is a double-based propellant.
 16. A method in accordance with claim 9 wherein each of the smaller chambers defined by the separator structure has a mandrel positioned therein, said mandrels being coated with a mold release agent, which mandrels following the casting and curing are removed to provide longitudinal voids in the respective free-standing propellant grains.
 17. A method in accordance with claim 16 wherein additional propellant is cast and cured in the longitudinal voids of the respective free-standing propellant grains.
 18. The method of claim 17 in which the spaces provided by removing the mandrels are coated with a mold release agent prior to casting additional propellant.
 19. The method of claim 17 in which the propellant upon curing shrinks on the order of about 2 percent to about 3 percent.
 20. The method of claim 19 in which the propellant is a polyester-based propellant.
 21. The method of claim 19 in which the propellant is a double-based propellant. 